The present invention relates to a pressure sensor and particularly to a pressure sensor of the type having a semiconductor diaphragm on which a diffused resistor is formed as a pressure-sensitive element using the piezoresistivity effect.
A variety of semiconductor pressure sensors have been proposed which use the piezoresistivity effect of a semiconductor material constituting the sensors, as shown in FIGS. 1, 2 and 3 of the accompanying drawings. These sensors designated by reference numeral 10, include a silicon diaphragm block 12 which is composed of a diaphragm 14 and a peripheral support 16 formed from a silicon chip using etching techniques. The diaphragm 14 includes diffused resistors 18 formed as a pressure-sensitive element on a front surface 14a thereof. The opposite surface 14b of the diaphragm 14 formed using the etching techniques is subjected to a fluid pressure to be measured.
A typical semiconductor pressure sensor 10 shown in FIG. 1 includes an alumina base 20 to which the diaphragm block 12 is bonded by means of a bonding material 22 such that the front surface 14a on which the resistors 18 are formed is disposed so as to be subjected to a vacuum 24 within a package 25 formed by the base 20 and a cap 26 which is sealingly secured to the base 20 and which covers the diaphragm block 12. A fluid pressure subject to measurement is introduced through an inlet pipe 28 connected to the base 20 and a hole 30 provided in the base 20 so as to arrive at the back surface 14b of the diaphragm 14.
A semiconductor pressure sensor shown in FIG. 2 includes a silicon diaphragm block 12 and a silicon support plate 32 bonded thereto so as to form a vacuum chamber 34 therebetween. In this case, the diaphragm block 12 and the support plate 32 are of the same linear coefficient of thermal expansion in order to prevent thermal stress which may otherwise be produced in the diaphragm 14 due to the difference in linear coefficient of thermal expansion between the diaphragm 14 and the alumina base 20 if the diaphragm block 12 should be directly bonded to the base 20.
The support plate 32 is secured at its center to the base 20 by means of a bonding material 36. A fluid subject to measurement is introduced through an inlet pipe 28, a hole 30 in the base 20 and the inside of the cap 26 so as to arrive at the front surface 14a of the diaphragm 14.
A pressure sensor shown in FIG. 3 is of the same structure as the sensor in FIG. 1 except that the cap 26 is provided with a hole 38 therein and an inlet pipe 40 connected to the cap so as to communicate with the hole. Two fluid pressures are introduced through the inlet pipe 28 and the hole 30 and through the inlet pipe 40 and the hole 38 so as to arrive at the opposite surfaces 14b and 14a, respectively, of the diaphragm 14 thereby making it possible to obtain the differential between the two fluid pressures.
FIG. 4 shows an enlarged cross-section of a pressure-sensitive silicon diaphragm block used in a prior art pressure sensor such as shown in FIGS. 1 to 3. In the example, the diaphragm block 12 includes an N type silicon base 140 which has on one surface diffused resistors 18 containing P type impurities, a silicon dioxide film 142 formed on the base 140 and on the diffused resistors 18, and aluminum leads 144 formed on the film 142 and electrically connected to the resistors 18. If this diaphragm block is sealingly accommodated within the package such that the surface of the diaphragm on which the diffused resistors are formed faces the vacuum within the package, there will no problem that the diffused resistors deteriorate. However, if the diffused resistors on the diaphragm are exposed to a fluid subject to measurement, as shown in FIGS. 2 and 3, which is required for providing connections to the resistors 18, or for obtaining the differential pressure, provision of only the silicon dioxide film 142 on the diffused resistors 18 will be insufficient to prevent external permeation of moisture and impurity ions (especially sodium ions), which may be contained in the fluid subject to measurement, between the diffused resistors 18 and the film 142 with the result that the electrical characteristics of the silicon interface tends to be unstable, as is well known; the resistors are liable to deteriorate due to the moisture and corrosive gases or the like, thereby resulting in a change in the output characteristic of the sensor